1. Field of the Invention
The present invention relates to a linear illumination device and, particularly, to a linear illumination device suitable for use as a light source of a contact type color image sensor.
2. Description of the Prior Art
Japanese Patent Application Laid-open No. H10-56577 discloses a construction of a conventional long linear illumination device for a color image sensor, which is schematically shown in FIG. 1. In FIG. 1, in order to obtain a color signal corresponding to an image of an original 9, each identical portion of the original is illuminated with a red (R), green (G) and blue (B) lights from R, G and B color light emitting diodes (LED""s) while switching these LED""s by a switch circuit 60 with a light emitting timing shown in FIG. 2.
That is, LED""s 21, 22 and 23 corresponding to R, G and B color lights, respectively, are arranged on an end surface of an optical waveguide member 11 and emit an illumination light 45 containing R, G and B color lights downward by guiding them through the optical waveguide member 11 while scattering them suitably. The original 9 disposed below the optical waveguide member 11 is illuminated with the illumination light 45 and a reflection light 47 reflected from the surface of the original 9 is incident on an image sensor 50, which is composed of a sensor array, through an optical system, etc., which is not shown. The image sensor 50 converts each color light into an electric signal with a light-electric conversion timing as shown in the bottom of FIG. 2, so that an electric color information for every scan line of the original is obtained.
In the disclosed prior art construction, in which three LED""s are arranged in the end face of the optical waveguide member 11, however, it is very difficult to design an optical system capable of uniformly illuminating the original with lights having different wavelengths, so that the utilization efficiency of light is lowered.
Further, since the reliability of the blue color light emitting LED is not sufficient at present and the supply thereof is not enough, the cost of the blue LED becomes very high. Consequently, the cost of the linear illumination device itself becomes high.
An object of the present invention is to provide an inexpensive linear illumination device capable of realizing an improvement of uniformity of a distribution of illumination light in a lengthwise direction of a linear light source thereof.
According to the present invention, a linear illumination device comprises an optical waveguide member for guiding three primary color lights incident on a light incident surface thereof and emitting them from a rectangular linear light emitting surface thereof, a point light source including at least one light emitting diode (referred to as LED, hereinafter), etc., provided on the optical waveguide member for supplying light through the light incident surface of the optical waveguide member into an interior thereof and a surface emitting light source including at least one light emitting element such as organic electro luminescent (referred to as EL, hereinafter) element provided on a surface portion of the optical waveguide member other than the light emitting surface thereof for reflecting back the light emitted from the point light source into the interior of the optical waveguide member.
A rear side electrode, that is a cathode electrode, of the EL element opposite to a transparent electrode, that is, an anode electrode, thereof is constituted with a light-reflecting layer of such as aluminum. An LED is employed as a red color light source and an EL element is employed as a blue color light source. As a green color light source, either LED or EL element is employed.
In a case where an LED for a red color light and two EL elements for a blue and green color lights are used, a linear illumination device is obtained by providing the red color LED on an end face of the optical waveguide member and the two EL elements on an upper surface of the optical waveguide member. Alternatively, it is possible to obtain the linear illumination device by providing a plurality of red color LED""s on the upper surface of the optical waveguide member and the two EL elements on respective opposite side surfaces of the optical waveguide member. In a case where a single EL element is used for blue color light, the linear illumination device is obtained by providing a red color LED and a green color LED on the end face of the optical waveguide member and the blue color EL element on the upper surface of the optical waveguide member. Alternatively, it is possible to obtain the linear illumination device by forming a plurality of openings in the blue color EL element provided on the upper surface of the optical waveguide member and arranging the red color LED""s and the green color LED""s in the openings alternately.
The EL element is preferably an organic thin film EL element including a transparent front electrode as an anode electrode on the side of the optical waveguide member, a metal rear electrode as a cathode electrode, which may be formed of aluminum and has a light reflecting function and an organic thin film light emitting layer provided between the transparent front electrode and the metal rear electrode.
The linear illumination device is constructed such that the point light source and a surface emitting light source emit three primary color lights sequentially. Further, the metal rear electrode of the EL element is formed to cover the upper and side surfaces of the optical waveguide member to thereby effectively reflect light in an interior of the optical waveguide member and guide it to the light emitting surface thereof.
The linear illumination device according to the present invention can use not only EL element but also LED for emitting blue color light. Therefore, it is possible to reduce the cost of linear illumination device when the LED is used as the blue color light source. Further, according to the present invention, LED or LED""s and EL element or elements are combined suitably. Therefore, it is possible to employ an inexpensive construction of linear illumination device. Further, since the EL element is a surface light emitting element, it is possible to illuminate a plane to be illuminated with high uniformity.
Further, since the light emitting area of the red color LED, which is a point light source, can be defined to a single point, the design of the optical waveguide member for uniformly diffusing light to a linear light becomes substantially easier compared with the conventional design for uniformly diffusing lights having different wavelengths and the uniformity of illumination can be improved compared with the conventional case.
Further, both the blue and green color lights may be emitted by the EL elements. Therefore, no unevenness of amounts of blue and green color lights occurs due to the rectangular parallelepiped optical waveguide member and a uniform illumination for an original to be illuminated can be obtained. Consequently, the light amount of illumination is uniformed, so that it becomes possible to improve the quality of image taken in by converting a reflected light reflected by the original illuminated by the linear illumination device into an electric signal by an image sensor.